Purge the cache of old entries. This reduces the risk of false positives from things like reused message IDs, but increases the risk of false negatives.

The ttl option specifies the "time to live": cache entries older than that will be purged. The default is one week. If the TTL is zero, then (just as you'd expect) items one second or older will be purged. If you specify a negative TTL, then the cache will be emptied completely.

The dump() method assumes that Perl's time() function returns seconds since the UNIX epoch, 00:00:00 UTC, January 1, 1970. The module will work on architectures with non-standard epochs, but the automated tests will fail.